Coalesced mode coupled cavity slow wave tube

ABSTRACT

A coupled cavity coalesced mode slow wave tube is disclosed. In the tube, the slow wave circuit is formed by a succession of cavity resonators coupled together via the intermediary of coupling slots. The coupling slots are dimensioned or tuned to have a resonant frequency substantially at the upper band edge frequency of the cavity mode to coalesce the slot mode and the cavity mode frequencies at the band edge, thereby increasing the passband of the circuit. At least two sets of such coupling slots are provided throughout the circuit. Each set of such slots is disposed with their centers in substantial alignment with a line substantially parallel to the axis of the beam. Provision of the plural sets of slots substantially reduces the slot impedance of the circuit, thereby substantially increasing the operating bandwidth of the circuit as compared to a similar circuit employing only one set of in-line coupling slots.

United States Patent James 1 June6, 1972 [54] COALESCED MODE COUPLEDCAVITY SLOW WAVE TUBE Bertram G. James, Redwood City, Calif.-

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Nov. 16, 1970 [21] App]. No.: 89,670

[72] Inventor:

[52] US. Cl. ..315/3.5, BIS/3.6, 333/31 A,

Power Travelling Wave Tubes by Gittins, Copyright 1965, pgs. 67- 77,8808 TK 7872'! 75 G5 Primary Examiner-H. K. Saalbach AssistantExaminer-Saxfield Chatmon, Jr. Attorney-Stanley Z. Cole ABSTRACT Acoupled cavity coalesced mode slow wave tube is disclosed. In the tube,the slow wave circuit is formed by a succession of cavity resonatorscoupled together via the intermediary of coupling slots. The couplingslots are dimensioned or tuned to have a resonant frequencysubstantially at the upper band edge frequency of the cavity mode tocoalesce the slot mode and the cavity mode frequencies at the band edge,thereby increasing the passband of the circuit. At least two sets ofsuch coupling slots are provided throughout the circuit. Each set ofsuch slots is disposed with their centers in substantial alignment witha line substantially parallel to the axis of the beam. Provision of theplural sets of slots substantially reduces the slot impedance of thecircuit, thereby substantially increasing the operating bandwidth of thecircuit as compared to a similar circuit employing only one set ofin-line coupling slots.

5 Claim, 6 Drawing Figures COALESCED MODE COUPLED CAVITY SLOW WAVE TUBEDESCRIPTION OF THE PRIOR ART l-leretofore, coupled cavity coalesced modeslow wave tubes have been built. In such prior art tubes, a single setof in-line coupling slots was provided for coupling the cavities of thecircuit together to form a coupled cavity slow wave circuit. Thecoupling slots of the one set were each tuned to the upper band edgefrequency of the cavity mode for coalescing the slot and cavity modes toincrease the operating bandwidth of the tube. Such a slow wave tube isdisclosed and claimed in copending US. patent application Ser. No.69,198 filed Sept. 3, 1970 and assigned to the same assignee as thepresent inventron.

One of the problems with this prior art tube has been that the operatingbandwidth, at 16 GHz was only on the order of 3 percent. It is desirableto increase the bandwidth of the tube to a value substantially in excessof 3 percent, as of percent.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved coupled cavity coalesced modeslow wave tube.

One feature of the present invention is the provision, in a coupledcavity coalesced mode slow wave tube, of at least two sets of in-linecoupling slots for coupling together successive cavities of the slowwave circuit, whereby the slot impedance is substantially reduced ascompared to a tube employing only one set of in-line coupling slots tosubstantially increase the operating bandwidth of the tube.

Another feature of the present invention is the same as the precedingfeature wherein the two sets of in-line coupling slots lie on oppositesides of the beam.

Other features and advantages of the present invention will becomeapparent upon persual of the following specification taken in connectionwith the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram, partlyin block diagram form, artly in line diagram form, and partly insectional view, depicting the prior art microwave tube,

FIG. 2 is an w-B diagram depicting the passbands for the cavity mode,slot mode and coalesced cavity and slot mode circuits,

FIG. 3 is a plot of normalized frequency vs. phase shift per cavitydepicting the dispersive characteristics of the in-line coalesced modeslow wave circuits for several slot impedances,

FIG. 4 is a plot of interaction gap impedance Z, vs. phase shift percavity depicting the characteristics for the coalesced mode circuit as afunction of slot impedance,

FIG. 5 is a schematic perspective line diagram depicting the coupledcavity slow wave circuit of the present invention, and

FIG. 6 is a plan view of a cavity end wall for a coupled cavity circuitof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thereis shown a prior art microwave coupled cavity slow wave circuit tube 1.In the microwave tube 1, an electron gun 2 forms and projects a beam ofelectrons 3 over an elongated beam path to beam collector 4 disposed atthe terminal end of the beam path 3. The coupled cavity slow wavecircuit 5 is disposed along the beam path intermediate the gun 2 and thecollector 4 for cumulative electromagnetic interaction with the beam toproduce a growing wave on the circuit which is extracted as an outputsignal.

The coupled cavity slow wave circuit 5 includes a plurality of cavityresonators 6 successively disposed along the beam path. Adjacentcavities 6 are coupled together via the intermediary of coupling slots 7disposed in the common wall between the adjacent cavities. Input waveenergy to be amplified is fed to the up-stream cavity 6 via an inputwaveguide 8 communicating with the up-stream cavity 6 via a couplingiris 9. The wave energy coupled onto the slow wave circuit 5 propagatesthrough the circuit for cumulative electromagnetic interaction with thebeam to produce a growing wave on the circuit. The amplified wave energyis extracted from the down-stream end of the slow wave circuit 5 viaoutput waveguide 1 l coupled to the down-stream cavity 6 via theintermediary of output iris 12. The output energy is coupled viawaveguide 11 to a suitable load, such as an antenna, not shown. In atypical example, the slow wave circuit, collector, and anode of the gunare all operated at ground potential, whereas the cathode emitter of thegun 2 is pulsed or operated at a negative potential via a suitable powersupply, such as pulser 13, which supplies the beam voltage.

Referring now to FIG. 2 there is shown the dispersive characteristic forthe coupled cavity coalesced mode slow wave circuit 5 of FIG. 1. Moreparticularly, the coupling slots 7 are each tuned for a frequency whichis coincident with the upper band edge frequency of the coupled cavitymode of the circuit. In the coalesced mode circuit, the geometriccenters of the coupling slots 7 are all in substantial axial alignmentwith a line parallel to the axis of and to one side of the beam path. Ina typical example, the slot mode would have a dispersive characteristicas indicated by line 14 of FIG. 2, whereas the coupled cavity mode wouldhave a dispersive charac teristic as indicated by line 15. However, whenthe resonant frequency of the slots is caused to be coincident with theupper band edge frequency of the cavity mode, the two modes coalesce toform a composite coalesced mode, as shown by curve 16.

The coalesced mode circuit has a much broader cold bandwidth, for thesecond space harmonic, than that of the noncoalesced coupled cavitycircuit. In addition, undesired band edge oscillations that wereheretofore encountered near the upper band edge frequency of thenon-coalesced coupled cavity mode have been eliminated in the coalescedmode circuit.

Referring now to FIGS. 3 and 4, the dispersive characteristic of thecoalesced in-line slow wave circuit and the gap impedance characteristicof such circuits are shown as a function of the total slot impedance percavity of the slow wave circuit. More particularly, FIG. 3 shows afamily of dispersive characteristics for the coalesced in-line circuitwith the slot impedance per cavity increasing from 1 ohm to l megaohms.In a typical microwave tube, the gap impedance can vary between 700 to1,000 ohms and for wide bandwidth it is desirable to operate with as lowa slot impedance per cavity as practical.

In the prior art coupled cavity coalesced mode circuit of FIG. -1dimensioned for operating in the band of frequencies centered at about35 GI-Iz, the lowest practicable slot impedance per cavity was on theorder of 1,000 ohms. It would be desirable to decrease the slotimpedance per cavity substantially to obtain broader bandwidth for thecircuit while maintaining as high a phase shift per cavity as possiblein order to obtain a circuit with relatively high thermal capacity.

Accordingly, it has been found that by providing two sets of couplingslots running longitudinally of the tube that the slot impedance percavity can be reduced by a factor of 4, i.e., from l,000 to 240 ohms,thereby substantially increasing the bandwidth of the tube fromapproximately 3 percent to approximately 10 percent in the Ka bandcentered at 35 GHz. Each set of the coupling slots is in-line, i.e.geometric centers of the slots in opposite cavity end walls fall on astraight line which is generally parallel to the beam axis. One set ofslots is positioned on one side of the beam and the other set of slotsis positioned on the opposite side of the beam. This configuration isindicated in FIG. 5 where a first set of inline coupling slots isindicated at 7 and the second set of in-line coupling slots is indicatedat 18.

Referring now to FIG. 6, there is shown, in plan view, a common wallbetween adjacent coupled cavities 6' with the coupling slots providedtherein. In this embodiment, the cavities 6 are of rectangular crosssection as opposed to the circular cross section of FIG. and thecoupling slots 7 and 18 rectilinear as opposed to being curved in FIG.5. In a typical example of a coalescedmode coupled cavity circuitutilizing two sets of coupling slots for operation at- 35 GHz andproviding approximately 10 percent operating bandwidth, the

coupling slots 7' and 18' are each 0.160 inch long with a height of0.015 inch and the cavities 6 have a height h of 0.216 inch and width wof 0.200 inch. The-spacing between adjacent end walls, in the axiallydirection of the cavities 6 is approximately 0.048 inch. Operation isobtained with a phase shift per cavity of approximately 1.5 to 1.6 7:-radians.

Although the coupled cavity in-line circuit of FIGS. 5 and 6 has beendescribed as used for forward wave interaction in a I traveling wavetube operating with a phase shift per period 7 between l.51r and 1.61rradians, it may also be used to adto be interpreted as illustrative notin a limiting sense.

What is claimed is:

1. In a microwave tube; means for projecting a beam of electrons over anelongated beam path; slow wave circuit means disposed along the beampath in electromagnetic energy exchanging relation with the beam; saidslow wave circuit means including, an array of cavity resonatorsarranged along the beam path for successive electromagnetic interactionwith the beam, adjacent ones of said cavity resonator having common endwalls, an array of wave energy coupling slot means disposed in thecommon end walls of said cavity resonators for wave energy couplingtogether the array of cavity resonators to define a slow wave circuithaving a cavity mode passband of frequencies associated with the coupledcavities and a slot mode passband of frequencies centered at a higherfrequency than the cavity mode passband and associated with the array ofslots, said slots being dimensioned to have a resonant frequency atsubstantially the upper band edge frequency of the cavity mode and thelow frequency band edge of the slot mode for increasing the width of thepassband of the composite slow wave circuit, THE IMPROVEMENT WHERElN,said coupling slots are comprised of at least two sets of slots, eachset of said slots being disposed in opposite end walls of each of saidcavities with their geometric centers lying on one side of the beam pathand falling substantially in a plane defined by the axis of the beampath and the centers of said coupling slots, whereby the total slotimpedance for each coupled cavity is substantially reduced forsubstantially increasing the operating bandwidth of the tube compared toa tube using only one set of similar coupling slots.

2. The apparatus of claim 1 wherein each set of coupling slots has thegeometric centers of the coupling slots positioned to be substantiallyon a straight line which is generally parallel to the beam axis and toone side of the beam axis.

3. The apparatus of claim-l wherein said coupled cavity slow wavecircuit means is a backward wave circuit for the fundamental spaceharmonic.

4. In a method for stabilized operation of a coupled cavity slow wavetube operating with cumulative electromagnetic interaction with the beamin the region of l.01r to 2.01r radians of phase shift per period of theslow wave circuit the steps of, dimensioning the coupling slots betweenadjacent cavity resonators of the circuit to have a slot mode ofresonance substantially at the upper band edge frequency of the cavitymode passband of the slow wave circuit to coalesce the low frequencypassband edge of the slot mode with the upper passband edge of thecavity mode, thereby substantially increasing the width of the tEassbandof the composite slow wave circuit and eliminating e stop band offrequencies between the cavity and slot modes of propagation, andpositioning two sets of the coupling slots in opposite end walls of eachof the coupled cavities with each set of coupling slots lying onopposite sides of the beam path and falling substantially in a planedefined by the axis of the beam and the geometric centers of thecoupling slots.

5. The method of claim 4 including the step of, positioning thegeometric centers of each set of coupling slots to lie substantially ina straight line generally parallel to the beam axis and to one side ofthe beam path. a

1. In a microwave tube; means for projecting a beam of electrons over anelongated beam path; slow wave circuit means disposed along the beampath in electromagnetic energy exchanging relation with the beam; saidslow wave circuit means including, an array of cavity resonatorsarranged along the beam path for successive electromagnetic interactionwith the beam, adjacent ones of said cavity resonator having common endwalls, an array of wave energy coupling slot means disposed in thecommon end walls of said cavity resonators for wave energy couplingtogether the array of cavity resonators to define a slow wave circuiThaving a cavity mode passband of frequencies associated with the coupledcavities and a slot mode passband of frequencies centered at a higherfrequency than the cavity mode passband and associated with the array ofslots, said slots being dimensioned to have a resonant frequency atsubstantially the upper band edge frequency of the cavity mode and thelow frequency band edge of the slot mode for increasing the width of thepassband of the composite slow wave circuit, THE IMPROVEMENT WHEREIN,said coupling slots are comprised of at least two sets of slots, eachset of said slots being disposed in opposite end walls of each of saidcavities with their geometric centers lying on one side of the beam pathand falling substantially in a plane defined by the axis of the beampath and the centers of said coupling slots, whereby the total slotimpedance for each coupled cavity is substantially reduced forsubstantially increasing the operating bandwidth of the tube compared toa tube using only one set of similar coupling slots.
 2. The apparatus ofclaim 1 wherein each set of coupling slots has the geometric centers ofthe coupling slots positioned to be substantially on a straight linewhich is generally parallel to the beam axis and to one side of the beamaxis.
 3. The apparatus of claim 1 wherein said coupled cavity slow wavecircuit means is a backward wave circuit for the fundamental spaceharmonic.
 4. In a method for stabilized operation of a coupled cavityslow wave tube operating with cumulative electromagnetic interactionwith the beam in the region of 1.0 pi to 2.0 pi radians of phase shiftper period of the slow wave circuit the steps of, dimensioning thecoupling slots between adjacent cavity resonators of the circuit to havea slot mode of resonance substantially at the upper band edge frequencyof the cavity mode passband of the slow wave circuit to coalesce the lowfrequency passband edge of the slot mode with the upper passband edge ofthe cavity mode, thereby substantially increasing the width of thepassband of the composite slow wave circuit and eliminating the stopband of frequencies between the cavity and slot modes of propagation,and positioning two sets of the coupling slots in opposite end walls ofeach of the coupled cavities with each set of coupling slots lying onopposite sides of the beam path and falling substantially in a planedefined by the axis of the beam and the geometric centers of thecoupling slots.
 5. The method of claim 4 including the step of,positioning the geometric centers of each set of coupling slots to liesubstantially in a straight line generally parallel to the beam axis andto one side of the beam path.